Improve tests build

Tests depend on the main build target
Add FLEX_HOME_TESTS property to tests target call, initialized with the current FLEX_HOME_TESTS value
Enable multiplatform support on the tests build itself

If I understand correctly, this also makes tests/graniteds_test/env.properties obsolete, however I have left it for now to be on the safe side. 

On my platform I get some erros while running the tests though the build says it is successful:

    [flexunit] FlexUnit test testResetEntityGDS667 in suite org.granite.test.tide.data.TestResetEntityGDS667 was ignored.
    [flexunit] FlexUnit test testEntityRefs in suite org.granite.test.tide.data.TestEntityRefs was ignored.
    [flexunit] FlexUnit test testMergeEntityMap in suite org.granite.test.tide.data.TestMergeEntityMap was ignored.
    [flexunit] FlexUnit test testCollectionEnumExtGDS588 in suite org.granite.test.util.TestCollectionEnumExtGDS588 was ignored.
    [flexunit] FlexUnit test testEjbPagedQuery in suite org.granite.test.tide.ejb.TestEjbClientPagedQuery had errors.
    [flexunit] FlexUnit test testEjbPagedQuery in suite org.granite.test.tide.ejb.TestEjbServerPagedQuery had errors.
    [flexunit] FlexUnit test testSpringPagedQueryController in suite org.granite.test.tide.spring.TestSpringPagedQueryController had errors.
    [flexunit] FlexUnit test testSpringComponentRestrict in suite org.granite.test.tide.spring.TestSpringComponentRestrict failed.
    [flexunit] FlexUnit test testSpringPagedQuery in suite org.granite.test.tide.spring.TestSpringClientPagedQuery had errors.
    [flexunit] FlexUnit test testSpringPagedQuery in suite org.granite.test.tide.spring.TestSpringServerPagedQuery had errors.
    [flexunit] FlexUnit test testGrailsPagedQuery in suite org.granite.test.tide.spring.TestGrailsPagedQuery had errors.
    [flexunit] FlexUnit test testSeamPagedQuery in suite org.granite.test.tide.seam.TestSeamClientPagedQuery had errors.
    [flexunit] FlexUnit test testServerSeamPagedQuery in suite org.granite.test.tide.seam.TestSeamServerPagedQuery had errors.
    [flexunit] FlexUnit test testInjectedCall in suite org.granite.test.tide.cdi.TestCdiInjectedCall was ignored.